FIG. 1 shows a schematic view of a parallel operational system of a conventional AC output inverter disclosed in U.S. Pat. No. 5,212,630. Referring to FIG. 1, a first inverter device 11 operates in parallel with a second inverter device 12, which has like construction, through an output bus 13, and supplies electric power to a load 14. The first inverter device 11 is mainly composed of an inverter body 110, a reactor 111 and condenser 112 serving as a filter. In order to operate the first and second inverter in parallel, a detection signal I1a is obtained from an output current 11 of the first inverter device 11 by a current transformer 120a, and a difference between the detection signal I1a and I2a similarly obtained from the second inverter device 12, that is a signal corresponding to cross current is obtained by a cross current detector 151. Then two orthogonal vectors EA and EB are generated by a phase shifter 150, and a reactive power corresponding power component and an active power component are obtained from the signal by arithmetic circuits 152, 153. A voltage control circuit 143 performs pulse width modulation for the inverter body 110 through a PWM circuit 140 based on signals from a voltage setting circuit 17 and a voltage feedback circuit 130, thereby controlling the internal voltage.
The above reactive power corresponding component is supplied as a supplementary signal to the voltage control circuit 143, and the internal voltage of the inverter body is adjusted that ΔQ becomes zero.
On the other hand, the active power corresponding component is input to a reference oscillator 156 through a PLL amplifier circuit 154, and the phase of the internal voltage of the inverter body is adjusted that ΔP becomes zero.
Since the voltage and phase are thus controlled that ΔQ and ΔP become zero, no cross current exists between the two inverters and stable load sharing is achieved.
However, the conventional parallel operational system of inverters has the following problems. First, since shared currents are balanced by controlling the phase and an average value of the internal voltage of the inverters, it is difficult to improve the response speed of control, and in particular, it is impossible to control instantaneous cross current. Secondly, since a filter is necessary to detect an active component and reactive component of the cross current separately, the cross current cannot be controlled at high speed. Therefore, there is a limit in applying the system to high-speed voltage control, for an example, instantaneous voltage control, which ensures that an output of the inverter a sine wave of high quality with little distortion. Thirdly, a current sharing bus is required to provide the instantaneous current signal, which is an analogy signal. Therefore, the sharing bus should have wide bandwidth, which makes the Electromagnetic Interference (EMI) easily disturb the stability of the system and it is impossible to use digital communication bus to transfer instantaneous current information.
It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.